In spirit of By-Tor, and considering I don't live in Pittsburgh anymore, I'll reveal the best place to park for a Steelers game :
Google Maps
Look to the left of Heinz Field and you'll see a street called Lighthill. To call it a street is really a joke, it's just a strip of land between a warehouse and a stadium lot. I have parked there for years and have never had a problem. It usually gets packed because other cars will see you parked there and will join in. Turns into a real good tailgating spot too since you got a stadium lot right there and plenty of room to setup whatever you want.
The one bad thing is you're right on the river, so it can get pretty cold in the winter. I parked there for the AFC Championship game vs New England a few years back. I got out of my car, opened up a Miller Lite bottle, took a big chug, set it down on my car because to grab my jacket out of my car, put on my jacket, grabbed my bottle of beer and it had froze solid.